FR2999760A1 - METHOD FOR ENCODING AND DECODING IMAGES, CORRESPONDING ENCODING AND DECODING DEVICE AND COMPUTER PROGRAMS - Google Patents

Abstract

The invention relates to a method for encoding at least one image comprising the steps of partitioning (C1, C2) the image into a plurality of blocks (CTU1, CTU2, CTUi, ..., CTUs) able to contain symbols belonging to a predetermined set of symbols, entropic coding of a current block from at least one symbol occurrence probability and predictive coding of a compression parameter (QP) associated with said current block. In the case where the current block is the first block to be encoded of a considered set of consecutive blocks to be encoded, the following steps are performed: • determination (C40a)), during said entropy coding step, probabilities of symbol appearance for said first current block, said probabilities being those which are determined at the end of the coding then decoding of the last block of another considered set of blocks, said other set immediately preceding, in the coding order blocks, said set considered, • assignment (C41a)), during said predictive coding step of a compression parameter associated with said first current block, a compression parameter prediction value which is independent of the value of the compression parameter associated with said last block.

Description

FIELD OF THE INVENTION The present invention relates generally to the field of image processing, and more specifically to the coding and decoding of images. digital images and digital image sequences. The invention can thus notably apply to video coding implemented in current video encoders (MPEG, H.264, etc.) or future ITU-T / VCEG (HEVC) or ISO / MPEG (HVC) encoders. BACKGROUND OF THE INVENTION The HEVC standard currently being developed and described in the document "B. Bross, W.-J. Han, J.-R. Ohm, GJ Sullivan, and T. Wiegand, "High efficiency video coding (HEVC) text specification draft 9," document JCTVC-K1003 of JCT-VC, Shanghai, ON, October 10-19, 2012 "is similar to the previous standard H .264, in that it uses block partitioning of the video sequence. However, the HEVC standard differs from the H.264 standard in that the partitioning implemented respects a tree structure called "quadtree". For this purpose, as shown in FIG. 1, a current image IN is partitioned a first time into a plurality of square blocks CTBi, CTB2, CTBi,..., CTBL, for example of size 64x64 pixels fflL). For an OT13 block; Given, it is considered that this block constitutes the root of a coding tree in which: a first level of leaves under the root corresponds to a first level of depth of partitioning of the block CTI3; for which block CTI3; has been partitioned a first time into a plurality of coding blocks, - a second level of sheets under the first level of sheets corresponds to a second level of block partitioning depth CTI3; for which some blocks of said plurality of coding blocks of the block OT13; partitioned a first time are partitioned into a plurality of coding blocks, ... - ... a kth level of sheets under the k-1th level of sheets which corresponds to a kth level of block partitioning depth CTI3; for which some blocks of said plurality of coding blocks of the block OT13; Partitioned k-1 times are partitioned one last time into a plurality of coding blocks. In a HEVC-compatible encoder, the iteration of the partitioning of the CTI3 block; is performed to a predetermined level of partitioning depth. At the end of the aforementioned successive partitioning of the block CTI3i, as represented in FIG. 1, the latter is finally partitioned into a plurality of coding blocks denoted C131, CB2, ..., CBj,..., CBm with En Referring to FIG. 1, for a given block CB j, it is considered that this block constitutes the root of a prediction and transformation tree of said block, for example of the discrete cosine transform (DOT) type. The prediction tree of a given block CBj is representative of how the block CBj is partitioned into a plurality of blocks which are called prediction blocks. For a prediction block considered and as in the H.264 standard, the aforementioned HEVC standard implements a prediction of pixels of said block with respect to pixels of at least one other block which belongs either to the same image (intra prediction ), or to one or more previous images of the sequence (inter prediction) that have already been decoded. Such prior images are conventionally referred to as reference images and are stored in memory at both the encoder and the decoder. Inter prediction is commonly referred to as motion-compensated prediction. At the end of the prediction of a block considered is delivered a predicted block. In accordance with the HEVC standard, during the transformation operation of a CBj coding block considered, the latter can be partitioned again into a plurality of smaller blocks TBi, TB2, ..., TB ,, ... , TBQ (1v <)) which are called transform blocks or sub-blocks. Such partitioning respects a "quadtree" type tree, called "residual quadtree", in which the leaves of the latter represent the transform blocks TBi, TB2,..., TI3,..., TBQ respectively. on different levels of partitioning depth. Each sub-block TB i, TB 2,..., TB i, TBQ contains the residue pixels representative of the difference between the pixels of the prediction block considered and the pixels of the current coding block CB i. In the example shown in FIG. 1, the prediction residue of the CBi coding block is for example partitioned into ten square sub-blocks of variable size TBi, TB2, TB3, TB4, TB5, TB8, TB7, TB8, TB9. TB10. Such partitioning is shown in dotted line in FIG.

The pixels of the prediction residue corresponding to each transform block TBi, TB2,..., TBQ can be signaled to the decoder as being all zero by a specific indicator called CBF (English abbreviation for "Coded Block Flag"). indicator is 0, the residue is interpreted as being zero in the prediction block considered, In the example represented in FIG. 1, the prediction blocks for which this indicator is equal to 0 are the sub-blocks TB 1 and TB 10. where this indicator is equal to 1, the residual blocks obtained are then transformed, for example using a DCT transform (discrete cosine transform) .In the example represented in FIG. 1, the prediction blocks for which this indicator is equal to 1 are the blocks TB2 to TB9, the coefficients of each of the transformed residual blocks are then quantized, for example using uniform scalar quantization, then coded by entropy coding Such steps are well known as such. More particularly, the quantization step uses a quantization step which is determined from a parameter called QP (abbreviation of "Quantization Parameter"). In the HEVC standard, it is possible to have a QP parameter that is the same for all the transform blocks of an image. In order to better adapt to the local characteristics of the image, it is also possible to modify the parameter QP for each coding block considered. This change is made as follows. For a current block, consider the first transform block that actually contains a residue (CBF = 1). With reference to FIG. 1, such a transform block is the block TB2. Modification information of the parameter QP is then transmitted to the decoder. In the HEVC standard, this information consists of a syntax element denoted QPdelta which is representative of the difference between the parameter QP of the previously coded then decoded block, called "predicted QP", and the parameter QP of the current block. It should be noted that the QP parameter of a current transform block is also used to determine the filtering strength of an anti-block or block effect filter (deblocking filter). In a manner known per se, such a filtering is applied to the edge of the transform blocks so as to reduce the effects of blocks which appear in an image, at the boundary of the transform blocks and the coding blocks. Particularly in the HEVC standard, different types of filtering can be considered, and the QP parameter is used to determine which type of filtering will be selected. The advantage of such an arrangement is, for example, to apply a more powerful filtering when the parameter QP is high (which corresponds to a strong compression of the block). It should be noted that even a block whose coefficients are zero (CBF = 0), for example the block P131 in FIG. 1, nevertheless has a parameter value QP which is predicted from the parameter value QP of FIG. previously encoded block then decoded, such a parameter that can be used in case of anti-block filtering. More particularly, the entropic coding can be implemented in an arithmetic coder called "CABAC" (Context Adaptive Binary Arithmetic Coder), introduced in the AVC compression standard (also known as ISO-MPEG4 part 10). and ITU-T H.264). This entropic coder implements different concepts: arithmetic coding: the coder, as initially described in the document J. Rissanen and G. G. Langdon, "Universal modeling and coding," IEEE Trans. lnform. Theory, vol. IT-27, pp. 12-23, Jan. 1981, uses, to encode a symbol, a probability of appearance of this symbol; - Adaptation to the context: it is a question of adapting the probability of appearance of the symbols to be coded. On the one hand, an on-the-fly learning is done. On the other hand, depending on the state of the previously coded information, a specific context is used for the coding. Each context corresponds to a probability of appearance of the symbol itself. For example, a context corresponds to a type of coded symbol (the representation of a coefficient of a residue, coding mode signaling, etc.) according to a given configuration, or a neighborhood state (for example the number of modes). "Intra" selected in the neighborhood, ...); binarization: a setting in the form of a sequence of bits of the symbols to be encoded is performed. Subsequently, these different bits are successively sent to the binary entropic coder. Thus, this entropic coder implements, for each context used, a system for learning on-the-fly probabilities with respect to the symbols previously coded for the context in question. In addition, the HEVC standard distinguishes several coding techniques. According to a first coding technique, the blocks of a current image are coded and then decoded sequentially according to a lexicographic order, for example according to a row-by-row path of the blocks, of the "raster-scan" type, starting from the block situated at the top left of the image to the block at the bottom right of the image. In the example shown in FIG. 1, the blocks CTBi to CTBL are coded and then decoded successively. In the case where the current block is the first block to be encoded of a considered set of consecutive blocks to be encoded, for example a line of blocks, it is proceeded to: - the determination, during the entropy coding of this first current block, symbol occurrence probabilities for said first current block, said probabilities being those determined for the last coded and decoded block of the preceding block line, - determining the QPdelta syntax element which is representative the difference between a predicted parameter value QP, which is that of the last coded and decoded block of the preceding block line, and a predetermined value of parameter QP that the coder wishes to associate with the first current block. Such a technique provides high image compression performance. However, the coding and the entropy decoding of a symbol being dependent on the state of the probability learned so far, the decoding of the symbols can be done in the same order as that used during the coding. Typically, the decoding can then be only sequential, thus preventing a parallel decoding of several symbols (for example to take advantage of multi-core architectures).

According to a second coding technique called Wavefront Parallel Processing (WPP), the blocks of a current image are grouped into a predetermined number of neighboring sets of blocks two by two. In the example shown in FIG. 1, said sets of blocks consist for example of each of the lines L1 to L6 of the image IN. The sets of blocks thus formed are encoded or decoded in parallel. Of course, such an encoding requires that the blocks located respectively above and above the right of the current block are available, so as to extract the data necessary for the prediction of said current block (decoded pixel values allowing predict the pixels of the current block in the intra mode, value of the motion vectors in the inter mode). According to this second coding technique, in the case where the current block is the first block to be encoded of a considered set of consecutive blocks to be coded, for example a block line, it is proceeded to: - the determination, during the coding entropic of this first current block, probabilities of appearance of symbol for said first current block, said probabilities being those which were determined at the end of the coding and decoding of the second block of the previous block line, - at the determining the syntax element denoted QPdelta which is representative of the difference between a predicted value of parameter QP, which is a predetermined value called QPslice, and a predetermined value of parameter QP that the encoder is intended to associate with the first current block.

In this way, it is possible to start the coding of a current block line without waiting for the last block of the previous line to be coded and then decoded. Such an arrangement has the advantage of accelerating the processing time of the coder / decoder and benefiting from a multiplatform architecture for the coding / decoding of an image. However, the compression performances obtained according to this second technique are not optimal considering the fact that the learning of the entropic coder CABAC probabilities is made slower because of the initialization of the probabilities at the beginning of the line.

In addition, in the documents mentioned at http: //phenix.int- evry.frict / doc end user / documents / 10 Stockholm / wg11 / JCTVC-J0032-v3.zip, it is proposed to either convert an image in which the blocks have been coded in WPP mode into an image in which the blocks have been coded in sequential mode, or conversely to convert an image in which the blocks have been coded in sequential mode into an image in which the blocks were coded in WPP mode. Converting WPP mode to sequential mode can improve compression performance at the expense of losing the ability to code / decode parallel lines. The conversion of the sequential mode to the WPP mode can make it possible to "parallelize" a stream of coded blocks which would have been received but which would not have been encoded in WPP mode, all this to the detriment of a slight loss in compression efficiency. . In the case, for example, of a conversion from the WPP mode to the sequential mode, it is first carried out with an entropy decoding of the blocks of the image which have been coded in WPP mode. The entropy decoded blocks are then entropically recoded in accordance with the sequential mode. The disadvantage of a conversion of the aforementioned type is that it operates only when the parameter QP is constant in the image, that is to say that the value of the parameter QP is identical for each block of the image. Indeed, when the QP parameter varies from one block to the other in a given stream of coded blocks, it is necessary, in addition to the aforementioned entropy decoding and entropic re-encoding steps, to decode the QPdelta syntax elements. represent the variations of the parameter QP, then modify the value of these syntax elements so that the QP parameter of each block is identical in WPP mode and in sequential mode. Such an arrangement is necessary in particular because the first block of a line inherits a QP parameter which is not the same depending on whether the encoder operates in sequential mode or in WPP mode, as has been the case. described above. In addition, assuming that the first block of a line does not contain any residue (CBP = 0), the decoding of the QPdelta syntax element can not be performed since this syntax element has not been transmitted. Thus, it may happen that the QP parameter of the first block of a line is different in WPP and non-WPP mode, which induces an anti-block filtering along the boundary of this first block which is different depending on whether the encoder operates in sequential mode or WPP mode. Consequently, said first block will be decoded differently in sequential mode and in WPP mode, which will make the image impossible to decode, since the first coded block of a line will not be the one expected. The values of the pixels of said first coded block being able to be reused for the intra prediction of the following blocks, but being different from the expected values, it is therefore the entire decoding process of the image that will be erroneous. OBJECT AND SUMMARY OF THE INVENTION One of the aims of the invention is to overcome disadvantages of the state of the art mentioned above.

For this purpose, an object of the present invention relates to a method of coding at least one image comprising the steps of: partitioning the image into a plurality of blocks able to contain symbols belonging to a predetermined set of symbols, entropy coding of a current block from at least one probability of appearance of a symbol; predictive coding of a compression parameter associated with the current block.

The coding method according to the invention is remarkable in that in the case where the current block is the first block to be encoded of a considered set of consecutive blocks to be coded, the following steps are carried out: entropy coding step, symbol appearance probabilities for the first current block, the aforementioned probabilities being those determined at the end of the coding then the decoding of the last block of another considered set of blocks, another set immediately preceding, in the coding order of the blocks, the set considered, - assigning, during the predictive coding step of a compression parameter associated with the first current block, a prediction value of compression parameter that is independent of the value of the compression parameter associated with the last aforementioned block. Such an arrangement makes it possible to produce an encoded image on which it is possible to carry out an error-free transcoding of sets of blocks which have been coded sequentially into sets of blocks which have been coded in parallel and vice versa, in particular in the case where a compression parameter, such as for example the quantization parameter, varies from one block to another in a considered image. Such an arrangement also makes it possible to benefit not only from the specific compression performance of the sequential mode but also from the speed of processing time of the encoder / decoder, once the image has been converted into WPP mode. Compression parameters, other than the aforementioned quantization parameter, may be considered, such as for example a parameter determining the number of intra-available mode, a parameter describing the maximum size of the motion vectors, etc.

In a particular embodiment, the blocks having previously been partitioned into sub-blocks, the assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the first block of the other set which immediately precedes , in the block coding order, the set considered. Such an arrangement has the advantage of making it possible to determine the prediction value of the compression parameter from the beginning of the processing of the previous set of blocks (for example, line of blocks greater than the current block line), which thus minimizes the timeout period to code the current block set (current block line). In another particular embodiment, the blocks having been previously partitioned into sub-blocks, the assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the second block of the other set that precedes immediately, in the block coding order, the set considered. Such an arrangement has the advantage of allowing some statistical learning of the value of the compression parameter on the data of the set of blocks preceding the current block. The efficiency of the prediction is thus made higher. In yet another particular embodiment, the blocks having been previously partitioned into sub-blocks, the assigned compression parameter prediction value is that of the compression parameter associated with the first sub-block of the first block of the image. Such an arrangement has the advantage of making a compromise between the waiting time for coding the set of current blocks and the statistical training mentioned above. In yet another particular embodiment, the assigned compression parameter prediction value is that of the compression parameter associated with the image. In the case where the image is encoded in accordance with HEVC, a block is a coding block and a subblock is a transform block. In yet another particular embodiment, the assigned compression parameter prediction value is that of the compression parameter associated with a set of blocks able to be encoded independently of other blocks of the image. Such a set of blocks able to be coded independently of other blocks of the image is called "slice". The compression parameter associated with a slice is, for example, the QPslice value parameter that is used in the HEVC standard when the image is divided into "slices". In general, a slice is a group of consecutive blocks that are encoded and then decoded independently, that is, they do not use any information from blocks outside the slice (for example, they do not can not use pixels that are outside the slice to make an intra prediction). It is known in the HEVC standard that each slice has a transmitted value called QPslice. For a slice considered, the first slice block takes the value QPslice as a compression parameter prediction value.

The slices considered are each representative of an area of the image having a certain homogeneity in terms of texture. Therefore, for each slice transform block containing a residue, the QPdelta syntax element will be lower than in the case of a high contrast image area. As a result, the QPdelta syntax element is less expensive to report to the decoder. Correlatively, the invention also relates to a coding device for at least one image, intended to implement the aforementioned coding method, such a device comprising: image partitioning means in a plurality of blocks able to contain symbols belonging to a predetermined set of symbols, - entropy coding means of a current block from at least one symbol appearance probability, - predictive coding means of a compression parameter associated with the current block. Such a coding device is remarkable in that in the case where the current block is the first block to be encoded of a considered set of consecutive blocks to be encoded: the entropy coding means determine the symbol appearance probabilities for the first current block, the aforementioned probabilities being those which were determined at the end of the coding then decoding of the last block of another considered set of blocks, this other set immediately preceding, in the coding order of the blocks, the considered set, - the predictive coding means of a compression parameter associated with the first current block affect a compression parameter prediction value which is independent of the value of the compression parameter associated with the last block. Correspondingly, the invention also relates to a method for decoding a representative stream of at least one coded picture, comprising the steps of: identifying in the stream a plurality of coded blocks, the aforementioned blocks being able to contain symbols belonging to a predetermined set of symbols, - entropy decoding of a current coded block from at least one probability of occurrence of symbol, - predictive decoding of a compression parameter associated with the current coded block. Such a decoding method is remarkable in that in the case where the current block is the first block to be decoded from a considered set of consecutive blocks to be decoded, the steps of: - determination, at the step of entropy decoding, probabilities of symbol occurrence for the first current block, the probabilities being those determined after the decoding of the last block of another considered set of blocks, this other set immediately preceding, in the the decoding order of the blocks, the set considered, - assigning, during the predictive decoding step of a compression parameter associated with the first current block, a compression parameter prediction value that is independent of that the compression parameter associated with the last aforementioned block. In a particular embodiment, the blocks having been partitioned into sub-blocks, the assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the first block of the other set immediately preceding, in the decoding order of the blocks, the set considered. In another particular embodiment, the blocks having been partitioned into sub-blocks, the assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the second block of the other set which immediately precedes , in the decoding order of the blocks, the set considered. In yet another particular embodiment, the blocks having been previously partitioned into sub-blocks, the assigned compression parameter prediction value is that of the compression parameter associated with the first sub-block of the first block of the image. In yet another particular embodiment, the assigned compression parameter prediction value is that of the compression parameter associated with a set of blocks able to be decoded independently of other blocks of the image. Correlatively, the invention relates to a device for decoding a stream representative of at least one coded picture, intended to implement the aforementioned decoding method, such a device comprising: identification means in the stream of a plurality of coded blocks, the aforementioned blocks being able to contain symbols belonging to a predetermined set of symbols; entropic decoding means of a current coded block from at least one probability of appearance of a symbol; predictive decoding means of a compression parameter associated with the current coded block. Such a decoding device is remarkable in that in the case where the current block is the first block to be decoded from a considered set of consecutive blocks to be decoded, the entropy decoding means determine probabilities of appearance of a symbol for the first current block, the aforementioned probabilities being those which were determined at the end of the decoding of the last block of another considered set of blocks, this other set immediately preceding, in the decoding order of the blocks, the set considered the predictive decoding means of a compression parameter associated with the first current block affect a compression parameter prediction value which is independent of that of the compression parameter associated with the last aforementioned block. The invention also relates to a computer program comprising instructions for executing the steps of the above coding or decoding method, when the program is executed by a computer.

Such a program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any another desirable form. Still another object of the invention is directed to a computer readable recording medium, and including computer program instructions as mentioned above. The recording medium may be any entity or device capable of storing the program. For example, the medium may include storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a magnetic recording means, for example a USB key or a hard disk. On the other hand, such a recording medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can be downloaded in particular on an Internet type network. Alternatively, such a recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute the method in question or to be used in the execution of the latter. The coding device, the decoding method, the decoding device and the aforementioned computer programs have at least the same advantages as those conferred by the coding method according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages will appear on reading two preferred embodiments described with reference to the figures in which: FIG. 1 represents an image coding scheme of the prior art, FIG. 2A represents the main steps of the coding method according to the invention, - Figure 2B shows in detail the coding implemented in the coding method of Figure 2A, - Figure 3A shows an embodiment of a coding device according to the invention, FIG. 3B represents a coding module of the coding device of FIG. 3A; FIG. 4 represents an image coding / decoding scheme according to sequential type; FIG. partitioning a block of the current image into sub-blocks, - Figure 6A represents the main steps of the decoding method according to the invention, - Figure 6B shows in detail the decoding implemented. In the decoding method of FIG. 6A, FIG. 7A represents an embodiment of a decoding device according to the invention, FIG. 7B represents a decoding module of the decoding device of FIG. 7A.

Detailed Description of an Embodiment of the Encoding Part An embodiment of the invention will now be described, in which the encoding method according to the invention is used to encode an image or sequence of images according to a flow. binary close to that obtained by an encoding conforming for example to the standard being developed HEVC. In this embodiment, the coding method according to the invention is for example implemented in a software or hardware way by modifications of an encoder initially conforming to the HEVC standard. The coding method according to the invention is represented in the form of an algorithm comprising steps C1 to 04 as represented in FIG. 2A. According to the embodiment of the invention, the coding method according to the invention is implemented in a coding device CO represented in FIG. 3A. As illustrated in FIG. 3A, such a coding device comprises an MEM memory CO comprising a buffer memory MT CO, a processing unit UT CO equipped for example with a microprocessor pl 'and driven by a computer program PG CO which sets implementing the coding method according to the invention. At initialization, the code instructions of the computer program PG CO are for example loaded into a RAM memory before being executed by the processor of the processing unit UT CO. The coding method shown in FIG. 2A applies to any current image of an SQ sequence of images to be encoded.

During a first step C1 shown in FIG. 2A, in a manner known per se, a partition is made of a current image belonging to the image sequence SQ IC1, IC1,. in a plurality of blocks CTUi, CTU2, CTUs for example of size 64x64 pixels (1S). Such a partitioning step is implemented by a partitioning software module MP shown in FIG. 3A, which module is driven by the microprocessor pl 'of the processing unit UT CO. The ICi image thus partitioned is shown in FIG. 4. It should be noted that for the purposes of the invention, the term "block" means coding unit (coding unit). This last terminology is notably used in the HEVC standard, for example in the document "B. Bross, W.-J. Han, J.-R. Ohm, G. J. Sullivan, and T. Wiegand, "High Efficiency Video Coding (HEVC) Text Specification Draft 6," JCTVC-H1003 of JCT-VC, San Jose CA, USA, February 2012.

In particular, such a coding unit groups together sets of pixels of rectangular or square shape, also called blocks, macroblocks, or sets of pixels having other geometrical shapes. Said blocks CTUi, CTU2, CTUi, ..., CTUs are able to contain one or more symbols, said symbols forming part of a predetermined set of symbols. During a step 02 shown in FIG. 2A, for a CTU current block; previously selected, the partitioning module MP partitions said block into a plurality of smaller blocks called encoding blocks YES, CU2, ..., CUf, ..., CUG with Such partitioning respects a tree of type "quadtree", as described above in the description. Other types of tree can of course be considered. Step 02 is reiterated for all the blocks CTUi, CTU2, ..., CTUs. An example of partitioning the CTU block; is shown in Figure 5. In this example, the CTU block; is partitioned according to a tree structure of "quadtree" type in four CUi CUio blocks. With reference to FIG. 2A, during a step 03, the blocks CTUi, CTU2, CTUi, ..., CTUs are coded according to a predetermined order of travel, which is for example of the raster scan type mentioned above. . In the example shown in FIG. 4, the blocks CTU1, CTU2, CTUi,..., CTUs are coded one after the other, from the left to the right, as indicated by the arrow PRS. Such an encoding is of the sequential type and is implemented by a UCO coding module as shown in FIG. 3A, which module is controlled by the microprocessor pl 'of the processing unit UT CO. In a manner known per se, the buffer memory MT CO of the coder CO is adapted to contain the symbol appearance probabilities such as progressively updated as the coding of a current block progresses. As shown in more detail in FIG. 3B, the UCO coding module comprises: a coding module predictive of a current block with respect to at least one previously coded and decoded block, denoted MCP; an entropy coding module of said current block by using at least one symbol appearance probability calculated for said previously coded and decoded block, denoted ECM. The predictive coding module MCP is a software module that is able to carry out a predictive coding of the current block, according to conventional prediction techniques, such as for example in the Intra mode and / or Inter mode. The entropic coding module MCE is CABAC type, but modified according to the present invention as will be described later in the description.

Alternatively, the entropic coding module MCE could be a Huffman coder known as such. In the example shown in FIG. 4, the UCO module codes the blocks of the first line LEi, from left to right. When it reaches the last block of the first line LEi, it goes to the first block of the second line LE2. When it reaches the last block of the second line LE2, it goes to the first block of the third line LE3. When it reaches the last block of the third line LE3, it goes to the first block of the fourth line LE4, and so on until the last block of the current picture IC1 is coded. Other types of course than the one just described above are of course possible. Thus, it is possible to cut the image ICi into several subimages called slices and to independently apply a division of this type on each sub-image. It is also possible for the UCO coding module to process not a succession of lines, as explained above, but a succession of columns. It is also possible to browse the rows or columns in one direction or the other. With reference to FIG. 2A, during a step 04, a flow F is constructed which is capable of being converted into WPP mode. The stream F is then transmitted by a communication network (not shown) to a remote terminal. This includes the decoder DO shown in FIG. 7A.

With reference to FIG. 2B, the different specific sub-steps of the invention, as implemented during the aforementioned coding step 03, will now be described in the UCO coding module represented in FIGS. 3B. During a step 031 represented in FIG. 2B, the coding module UCO selects as the current block the first sub-block to be coded from a block CTU; a line LE, - current shown in Figure 4, such as for example the first line LEi. In the following description, this sub-block will be called block. During a step 032 shown in FIG. 2B, the UCO coding module tests whether the current block is the first block (located at the top left of the image) ICi which has been cut into blocks, then under blocks in the aforementioned step C1. If this is the case, during a step 033 represented in FIG. 2B, the entropic coding module MCE initializes the symbol appearance probabilities.

During this same step 033 shown in Figure 2B, it is proceeded to the initialization of a predicted compression parameter value. In the example shown, the compression parameter considered is the quantization parameter QP. According to HEVC, the predicted QP parameter value is initialized to the QPslice value.

If following the aforementioned step 032, the current block is not the first block of the image IC 1, it is tested, during a step 039 which will be described later in the following description, if the current block is the first block of the line LEn, considered. During a step 034 represented in FIG. 2B, the first current block CUi of the first line LE1 represented in FIG. 4 is coded. Such a step 034 comprises a plurality of sub-steps 034a) to 034j). which will be described below. During a first substep 034a) represented in FIG. 2B, the predictive module MCP of FIG. 3B proceeds to predictive coding of the current block CUi by known intra and / or inter prediction techniques, during which the block CUi is predicted with respect to at least one block previously coded and then decoded.

It goes without saying that other intra prediction modes as proposed in the HEVC standard are possible. The current block CUi may also be subjected to predictive coding in inter mode, during which the current block is predicted with respect to a block resulting from a previously coded picture then decoded. Other types of prediction are of course conceivable. Among the possible predictions for a current block, the optimal prediction is chosen according to a distortion flow criterion well known to those skilled in the art. Said aforementioned predictive coding sub-step makes it possible to construct a predicted CUpi block which is an approximation of the current block CUi. The information relating to this predictive coding will subsequently be written in the stream F transmitted to the decoder DO. Such information notably includes the type of prediction (inter or intra), and if appropriate, the intra prediction mode, the type of partitioning of a block or macroblock if the latter has been partitioned, the image index of reference and displacement vector used in the inter prediction mode. This information is compressed by the CO encoder. During a subsequent substep C34b) shown in FIG. 2B, the predicted block CUpi of the current block CUi is subtracted to produce a residue block CUri. During a subsequent substep C34c) shown in FIG. 2B, the residue block CUri is partitioned into transform blocks TUi, TU2, ..., TUA, ..., TUE, with 1 1 KR. In the example shown in FIG. 5, the residue block CUri has been partitioned into four transform blocks TUi, TU2, TU3, TU4. During a subsequent substep 034d) shown in FIG. 2B, each transform block of the residue block CUri is transformed according to a conventional direct transformation operation such as, for example, a discrete cosine transformation of the type DOT, to produce a transformed CUTI block. During a following sub-step 034e) represented in FIG. 2B, the transformed block CUti is quantized according to a conventional quantization operation, such as, for example, a scalar quantization which uses a quantization parameter QP, such as this is the case in the HEVC standard. A block of quantized coefficients CUqi is then obtained. During a subsequent substep 034f) shown in FIG. 2B, if the current transform block TUr is the first transform block of the CTU current block; which contains a residue (CBF = 1), such as the block TU1 in FIG. 5, the coding of the QPdelta syntax element which is representative of the difference between the parameter QP of the previously coded sub-block then decoded is carried out , called "predicted QP", and the QP parameter of the current sub-block.

During a subsequent substep 034g) shown in FIG. 2B, the entropic coding of the quantized coefficient block CUqi is carried out. In the preferred embodiment, it is a CABAC entropic coding. Such a step consists in: a) reading the symbol (s) of the predetermined set of symbols which are associated with said current block, b) associating digital information, such as bits, with the (x) symbol (s) read (s) ). During a subsequent substep 034h) shown in FIG. 2B, dequantization of the CUqi block is carried out according to a conventional dequantization operation, which is the inverse operation of the quantization performed in step 034e). A block of dequantized coefficients CUDqi is then obtained. During a following substep 034i) shown in FIG. 2B, the inverse transformation of the dequantized coefficient block CUDqi is carried out which is the inverse operation of the direct transformation performed in step 034d) above. . A decoded residue block CUDri is then obtained. During a subsequent substep 034j) shown in FIG. 2B, the decoded block CUDi is constructed by adding to the predicted block CUpi the decoded residue block CUDri. It should be noted that this last block is the same as the decoded block obtained at the end of the image decoding process ICi which will be described later in the description. The decoded block CUDi is thus made available for use by the UCO coding module shown in FIGS. 3A and 3B.

At the end of the aforementioned coding step 034, the entropic coding module MCE as represented in FIG. 3B contains all the probabilities such as progressively updated as the first block is coded. These probabilities correspond to the different elements of possible syntaxes and to the different coding contexts associated. During a next step 035 shown in FIG. 2B, the decoded block CUDi is filtered by means of an anti-block filter in accordance with the HEVC standard. In particular, the anti-block filter uses the quantization parameter QP of the transform block TUr, which is current to determine the filtering force to be applied. In a next step 036 shown in FIG. 2B, the UCO coding module tests whether the current block of the newly-coded line LEm is the last block of the IC1 image.

If the current block is the last block of the image IC 1, during a subsequent step 037 shown in FIG. 2B, the coding method is terminated. If this is not the case, in the next step 038 shown in FIG. 2B, the next block CUf to be coded is selected according to the run command represented by the arrow PRS on FIG. FIG. 4. During a next step 039 represented in FIG. 2B, it is tested whether the current block is the first block of the line LEm considered. If this is the case, during an entropy coding step 040a) shown in FIG. 2B, the MCE entropy coding module of FIG. 3B starts coding the first current block of the line LEm considered with the probabilities of appearance of symbols calculated at the end of the coding and decoding of the last sub-block of the last block CTUd (1 c: IS) of the previous line LEm-i. With reference to FIG. 2B, during a step 041a) of predictive coding of a compression parameter associated with said first current block, such as, for example, the quantization parameter QP, the predictive coding module MCP of FIG. 3B assigns a compression parameter prediction value which is independent of the value of the compression parameter associated with said last sub-block of the last block CTUd (1eS) of the previous line LErn-i. According to a first variant of step 041a), the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block of the first block CTU of the preceding line LErn-i. With reference to FIG. 4, if the current block is, for example, the first sub-block of the block 0TU17 of the third line LE3, the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block of the first block CTU9 of the second line LE2.

According to a second variant of step 041a), the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block of the second block CTU of the preceding line LEm_i. With reference to FIG. 4, if the current block is, for example, the first sub-block of the block 0TU17 of the third line LE3, the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block the second block CTUi, the second line LE2. According to a third variant of step C41a), the prediction value of the quantization parameter QP affected is that of the quantization parameter associated with the first sub-block of the first block CTU of the image ICi. With reference to FIG. 4, if the current block is for example the first sub-block of the block 0TU17 of the third line LE3, the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the first sub-block of the first block CTUi of the first line LEi.

According to a fourth variant of step 041a), in the case where the image ICi has been divided into several slices, the prediction value of the quantization parameter QP assigned to the first current block is the value QPslice of the slice in which it is located. said first current block. Following step 041a), the current block is coded and then decoded by iteration of the steps 034 to 037 described above. If following the step 039 above, the current block is not the first block of the line LEm considered, it is proceeded, during a step 040b) of entropy coding shown in Figure 2B, to the reading probabilities from the last previously coded and decoded sub-block that is on the same line LEn, as the current block to be encoded, that is to say the last coded and decoded sub-block of the block CTU which is located immediately to the left of the current block. Such a step is performed by the entropic coding module MCE shown in FIG. 3B. With reference to FIG. 2B, it is then proceeded during a step C41b) of predictive coding of a compression parameter associated with said current block, such as, for example, the quantization parameter QP, with the assignment of a parameter prediction value QP to the value of the parameter QP of the last previously coded and decoded sub-block which is on the same line LEn, as the current block to be encoded, that is to say the last sub-block coded then decoded CTU block which is located immediately to the left of the current block. Such a step is performed by the predictive coding module MCP shown in FIG. 3B.

Following step C41b), the current block is coded and then decoded by iteration of the steps 034 to 037 described above. The coding steps which have just been described above are implemented for all the blocks CTUi, CTU2, CTU s to be coded of the current image ICi considered.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE DECODING PART An embodiment of the decoding method according to the invention will now be described, in which the decoding method is implemented in a software or hardware way by modifying an initially conforming decoder. to the HEVC standard. The decoding method according to the invention is represented in the form of an algorithm comprising steps D1 to D4 as represented in FIG. 6A. As illustrated in FIG. 7A, a decoder according to the invention comprises a memory MEM MEM comprising a buffer memory MT DO, a processing unit UT DO equipped for example with a microprocessor pl 'and driven by a computer program PG DO which implements the decoding method according to the invention. At initialization, the code instructions of the computer program PG DO are for example loaded into a RAM before being executed by the processor of the processing unit UT DO. The decoding method shown in FIG. 6A applies to any current image of an SQ sequence of images to be decoded.

For this purpose, information representative of the current image IC 1 to be decoded is identified in the stream F received at the decoder. With reference to FIG. 6A, the first decoding step D1 is the identification in said stream F of CTUi, CTU2, CTUi, ..., CTUs previously coded in accordance with the PRS path shown in FIG. 4.

With reference to FIG. 6A, the second decoding step D2 is the decoding of each of said L subsets LE1, LE2,..., LE ,,,..., LEL of blocks, the blocks of a sub-set. considered together being able to be decoded according to the predetermined sequential order of PRS, as shown in FIG. 4. In the example shown in FIG. 4, the blocks of a current subset LEm are decoded one after the other. others, from left to right, as indicated by the PRS arrow. At the end of step D2, the decoded block subsets LEDi, LED2, ..., LEDm,..., LEDL are obtained. Such a decoding is of sequential type and, consequently, is carried out using a single UDO decoding module as shown in FIG. 7A, said module being driven by the microprocessor uP of the processing unit UT DO . In a manner known per se, the buffer memory MT DO of the decoder DO is adapted to contain the symbol appearance probabilities such as progressively updated as a current block is decoded. As shown in more detail in FIG. 7B, the decoding module UDO comprises: an entropy decoding module of said current block by learning at least one symbol appearance probability computed for at least one previously decoded block, denoted MDE a decoding module predictive of a current block with respect to said previously decoded block, denoted CDM.

The predictive decoding module MDP is able to perform a predictive decoding of the current block, according to conventional prediction techniques, such as for example in lntra and / or Inter mode. The entropy decoding module MDE is CABAC type, but modified according to the present invention as will be described later in the description. Alternatively, the entropy decoding module MDE could be a Huffman decoder known as such. In the example shown in Figure 4, the UDO module decodes the blocks of the first line LEi, from left to right. When it reaches the last block of the first line LEi, it goes to the first block of the second line LE2. When it reaches the last block of the second line LE2, it goes to the first block of the third line LE3. When it reaches the last block of the third line LE3, it goes to the first block of the fourth line LE4, and so on until the last block of the current picture IC1 is decoded. Other types of course than the one just described above are of course possible. Thus, it is possible to cut the image ICi into several subimages called slices and to independently apply a division of this type on each sub-image. It is also possible for the UDO decoding module to process not a succession of lines, as explained above, but a succession of columns. It is also possible to browse the rows or columns in one direction or the other. Referring to FIG. 6A, the third decoding step D3 is the reconstruction of a decoded picture IDi from each decoded subset LEDi, LED2,..., LED,... decoding step D2. More specifically, the decoded blocks of each decoded subset LEDi, LED2,..., LED ,,..., LEDL are transmitted to an image reconstruction unit URI as represented in FIG. 7A, said unit being driven by the microprocessor pl 'of the processing unit UT DO. During this step D3, the URI unit writes the decoded blocks in a decoded image as these blocks become available.

During a fourth decoding step D4 shown in FIG. 6A, a fully decoded IDi image is delivered by the URI unit shown in FIG. 7A. We will now describe, with reference to FIG. 6B, the different specific sub-steps of the invention, as implemented during the aforementioned sequential decoding step D2, in the UDO decoding module represented in FIGS. 7A. and 7B. During a step D21 represented in FIG. 6B, the decoding module UDO selects as the current block the first sub-block to be decoded from a block CTU; a line LE, - current shown in Figure 4, such as for example the first line LEi. In the following description, this sub-block will be called block. During a step D22 shown in FIG. 6B, the decoding module UDO tests whether the current block is the first block (located at the top left) of the image IC 1 that has been previously coded. If this is the case, during a step D23 shown in FIG. 6B, the entropy decoding module MDE initialises the symbol appearance probabilities. Such an initialization is identical to that performed at the coding in step 033 shown in FIG. 2B.

During this same step D23 shown in Figure 6B, it is proceeded to the initialization of a predicted compression parameter value. In the example shown, the compression parameter considered is the quantization parameter QP. According to HEVC, the predicted QP parameter value is initialized to the QPslice value.

If following the aforesaid step D22, the current block is not the first block of the image ICi, it is tested, during a step D29 which will be described later in the following description, if the current block is the first block of the line LEn, considered. During a step D24 shown in FIG. 6B, the first current block CUi of the first line LE1 shown in FIG. 4 is decoded. Such a step D24 comprises a plurality of substeps D24a) to D24f). which will be described below.

During a first substep D24a) shown in FIG. 6B, the entropy decoding module MDE of FIG. 7B proceeds to the entropy decoding of the syntax elements linked to the current block. Such a step consists mainly of: a) reading the bits contained in the stream that are associated with said first line LEI, b) reconstructing the symbols from the read bits. More precisely, the syntax elements related to the current block are decoded by the entrapment decoding module MDE CABAC as represented in FIG. 7B. The latter decodes the bit stream F of the compressed file to produce the syntax elements, and at the same time updates its probabilities so that, at the moment when the latter decodes a symbol, the probabilities of appearance of this symbol are identical to those obtained during the coding of this same symbol during the aforementioned entropy coding step 034g) shown in Figure 2B. During a subsequent substep D24b) shown in FIG. 6B, the predictive decoding module MDP of FIG. 7B proceeds to the predictive decoding of the current block CUi by known intra and / or inter prediction techniques during which the CUi block is predicted with respect to at least one previously decoded block. It goes without saying that other intra prediction modes as proposed in the HEVC standard are possible. The current block CU i can also be subjected to predictive decoding in inter mode, during which the current block is predicted with respect to a block resulting from a previously decoded picture. Other types of prediction are of course conceivable. Among the possible predictions for a current block, the optimal prediction is chosen according to a distortion flow criterion well known to those skilled in the art. Said aforementioned predictive decoding sub-step makes it possible to construct a predicted CUpi block which is an approximation of the current block CUi. During a following substep D24c) shown in FIG. 6B, if the current transform block TUr is the first transform block of the current block CTUi that contains a residue (CBF = 1), such as block TU1 in FIG. 5, the QPdelta syntax element is decoded and the value of the QP parameter of the current sub-block is calculated by adding the decoded QPdelta syntax element to the value of the QP parameter of the sub-block previously decoded, called "QP predicts">. During a subsequent substep D24d) shown in FIG. 6B, dequantization of the quantized block CUqi is carried out according to a conventional dequantization operation which is the inverse operation of the quantization performed in the aforementioned step 034e) shown in Figure 2B, to produce a decoded dequantized block CUDti. During a subsequent substep D24e) shown in FIG. 6B, the inverse transformation of the dequantized block CUDti, which is the inverse operation of the direct transformation carried out in the above-mentioned step C34d) represented in FIG. 2B. A decoded residue block CUDri is then obtained. In the course of a subsequent substep D24f) shown in FIG. 6B, the decoded block CUDi is constructed by adding or block CUpi decoded the decoded residue block CUDri. The decoded block CUDi is thus made available for use by the UDO decoding module shown in FIGS. 7A and 7B. At the end of the aforementioned decoding step D24, the entropy decoding module MDE as represented in FIG. 7B contains all the probabilities such as progressively being updated as the first block is decoded. These probabilities correspond to the different possible syntax elements and the different decoding contexts associated with them. During a next step D25 shown in FIG. 6B, the decoded block CUDi is filtered by means of an anti-block filter in accordance with the HEVC standard. In particular, the anti-block filter uses the quantization parameter QP of the transform block TUr, which is current to determine the filtering force to be applied.

During a next step D26 shown in FIG. 6B, the decoding module UDO tests whether the current block CUDi that has just been decoded is the last block contained in the stream F. If this is the case, during In a step D27 shown in FIG. 6B, the decoding method is terminated. If this is not the case, during step D28 shown in FIG. 6B, the next block CUf to be decoded is selected in accordance with the run command represented by the arrow PRS in FIG. 4.

During a next step D29 shown in FIG. 6B, it is tested if the current block is the first block of the line LEm considered. If this is the case, during an entropy decoding step D30a), the entropy decoding module MDE of FIG. 7B begins decoding the first current block of the line LEm considered with the probabilities of appearance of symbols calculated in FIG. the result of the decoding of the last sub-block of the last block CTUd (1 * :: IS) of the previous line LErn-i. With reference to FIG. 6B, during a predicative decoding step D31a) of a compression parameter associated with said first current block, such as, for example, the quantization parameter QP, the predictive decoding module MDP of FIG. 7B assigns a compression parameter prediction value which is independent of the value of the compression parameter associated with said last sub-block of the last block CTUd (1eS) of the previous line LErn-i. According to a first variant of step D31a), the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block of the first block CTU of the previous line LErn-i. With reference to FIG. 4, if the current block is, for example, the first sub-block of the block 0TU17 of the third line LE3, the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block of the first block CTU9 of the second line LE2. According to a second variant of step D31a), the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block of the second block CTU of the preceding line LE ,, _ 1. With reference to FIG. 4, if the current block is for example the first sub-block of the block CTU17 of the third line LE3, the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the last sub-block the second block CTU10 of the second line LE2. According to a third variant of step D31a), the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the first sub-block of the first block CTU of the image ICi. With reference to FIG. 4, if the current block is for example the first sub-block of the block CTU17 of the third line LE3, the prediction value of the quantization parameter QP assigned is that of the quantization parameter associated with the first sub-block of the first block CTUi of the first line LEi. According to a fourth variant of the step D31a), in the case where the image ICi has been split into several slices, the prediction value of the quantization parameter QP assigned to the first current block is the value QPslice of the slice in which it is located. said first current block. Following step D31a), the current block is decoded by iteration of the steps D24 to D27 described above. If following the aforementioned step D29, the current block is not the first block of the line LEn, considered, it is proceeded, during an entropy decoding step D30b) shown in FIG. the reading of the probabilities coming from the last previously decoded sub-block which is on the same line LEn, as the current block to be decoded, that is to say the last decoded sub-block of the block CTU which is situated immediately on the left of the current block.

Such a step is implemented by the entropy decoding module MDE shown in FIG. 7B. With reference to FIG. 6B, a predictive decoding step of a compression parameter associated with said current block, such as, for example, the quantization parameter QP, is then carried out during a predefined step D31b). a parameter prediction value QP to the value of the parameter QP of the last previously decoded sub-block which is on the same line LEn, as the current block to be decoded, that is to say the last decoded sub-block of the CTU block that is located immediately to the left of the current block. Such a step is implemented by the predictive decoding module MDP shown in FIG. 7B. Following step D31b), the current block is decoded by iteration of the steps D24 to D27 described above.

The decoding steps that have just been described above are implemented for all the blocks CTUi, CTU2, CTUi, ..., CTUs to be decoded of the current image ICi considered. It goes without saying that the embodiments which have been described above have been given for purely indicative and non-limiting purposes, and that many modifications can easily be made by those skilled in the art without departing from the scope. of the invention.

Claims (16)

REVENDICATIONS1. A method for encoding at least one image (ICi) comprising the steps of: - partitioning (Cl, 02) of the image into a plurality of blocks (CTUi, CTU2, CTUi, ..., CTUs) able to contain symbols belonging to a predetermined set of symbols, - entropy coding of a current block from at least one symbol occurrence probability, - predictive coding of a compression parameter (QP) associated with said current block, said method coding method being characterized in that in the case where the current block is the first block to be coded of a considered set (LE, -,) of consecutive blocks to be coded, the steps of: - determination (C40a) are carried out) during said entropy encoding step, symbol occurrence probabilities for said first current block, said probabilities being those determined after coding and then decoding the last block of another set (LE, -, _ i) considered blocks, said other set pr immediately yielding, in the coding order of the blocks, said considered set, - assignment (C41a)), during said step of predictive coding of a compression parameter associated with said first current block, with a parameter prediction value compression which is independent of the value of the compression parameter associated with said last block. 2. A coding method according to claim 1, in which said blocks having been partitioned into sub-blocks, said assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the first block of said other set. (LE ,, _ 1) immediately preceding, in the order of coding blocks, said set considered. 3. coding method according to claim 1, wherein said blocks having been previously partitioned into sub-blocks, said assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the second block of said other together (LE ,, _ 1) immediately preceding, in the block coding order, said set considered. 4. Encoding method according to claim 1, wherein said blocks having been previously partitioned into sub-blocks, said assigned compression parameter prediction value is that of the compression parameter associated with the first sub-block of the first block of the block. 'picture. The coding method as claimed in claim 1, in which said assigned compression parameter prediction value is that of the compression parameter (QPslice) associated with a set of blocks able to be coded independently of other blocks of the image. . 6. Device coding (CO) of at least one image, intended to implement the coding method according to any one of claims 1 to 5, said device comprising: - means (MP) partitioning the image in a plurality of blocks (CTU) able to contain symbols belonging to a predetermined set of symbols, - entropy coding means (MCE) of a current block from at least one probability of appearance of a symbol, means (MCP) for predictive coding of a compression parameter (QP) associated with said current block, said coding device being characterized in that, in the case where the current block is the first block to be coded of a set considered of consecutive blocks to be encoded: the entropy coding means determine symbol appearance probabilities for said first current block, said probabilities being those which were determined at the end of the coding then decoding of the last block of u n another considered set of blocks, said other set immediately preceding, in the coding order of the blocks, said set considered, - the predictive coding means of a compression parameter associated with said first current block affect a parameter prediction value compression which is independent of the value of the compression parameter associated with said last block. A computer program comprising instructions for implementing the encoding method according to any one of claims 1 to 5, when executed on a computer. A computer-readable recording medium on which is recorded a computer program comprising instructions for performing the steps of the encoding method according to any one of claims 1 to 5, when said program is executed by a computer. 9. A method for decoding a stream (F) representative of at least one coded picture, comprising the steps of: - identifying (D1) in said stream of a plurality of coded blocks, said blocks being able to contain symbols belonging to a predetermined set of symbols, - entropy decoding of a current coded block from at least one symbol occurrence probability, - predictive decoding of a compression parameter (QP) associated with said current coded block, said decoding method being characterized in that in the case where the current block is the first block to be decoded from a considered set of consecutive blocks to be decoded, the steps of: - determination (D30a)) are carried out during said step entropy decoding method, probabilities of symbol occurrence for said first current block, said probabilities being those determined after the decoding of the last block of another set considered blocks, said other set immediately preceding, in the decoding order of the blocks, said considered set, - assignment (D31a)), during said predictive decoding step of a compression parameter associated with said first current block, by a a compression parameter prediction value which is independent of that of the compression parameter associated with said last block. The decoding method according to claim 9, wherein said blocks having been partitioned into sub-blocks, said assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the first block of said other set. immediately preceding, in the decoding order of the blocks, said set considered. 11. A decoding method according to claim 9, wherein said blocks having been partitioned into sub-blocks, said assigned compression parameter prediction value is that of the compression parameter associated with the last sub-block of the second block of said other set. immediately preceding, in the decoding order of the blocks, said set considered. 12. Decoding method according to claim 9, wherein said blocks having been previously partitioned into sub-blocks, said assigned compression parameter prediction value is that of the compression parameter associated with the first sub-block of the first block of the block. 'picture. 13. Decoding method according to claim 9, wherein said assigned compression parameter prediction value is that of the compression parameter (QPslice) associated with a set of blocks able to be decoded independently of other blocks of the image. 14. Device for decoding a stream (F) representative of at least one coded picture, intended to implement the decoding method according to any one of claims 9 to 13, said device comprising: - means (EX ID) in said flow of a plurality of coded blocks, said blocks being able to contain symbols belonging to a predetermined set of symbols, - entropic decoding means of a coded block current from at least a symbol appearance probability, - predictive decoding means of a compression parameter (QP) associated with said current coded block, said decoding device being characterized in that in the case where the current block is the first block to decoding from a considered set of consecutive blocks to be decoded, - the entropy decoding means (MDE) determines symbol occurrence probabilities for said first current block, said probabilities being those which were determined at the end of the decoding of the last block of another considered set of blocks, said other set immediately preceding, in the decoding order of the blocks, said set considered, - the means (MDP) of decoding prediction of a compression parameter associated with said first current block assigns a compression parameter prediction value which is independent of that of the compression parameter associated with said last block. 15. Computer program comprising instructions for implementing the decoding method according to any one of claims 9 to 13, when executed on a computer. A computer-readable recording medium on which a computer program is recorded including instructions for performing the steps of the decoding method according to any one of claims 9 to 13, when said program is executed by a computer. 10 15 20 25 30